The invention relates to an arrangement in a microwave oven for feeding microwave energy from a microwave source mounted external of an oven cavity bounded by a plurality of conductive walls into the interior of the oven cavity. More particularly, it relates to a feeding arrangement for an oven cavity that is excited in a conventional manner according to the well-known "multiresonance" principle.
Several known feeding arrangements for multiresonant oven cavities are based upon a movable metallic antenna that projects into the oven cavity and that is coupled, usually through a waveguide of TE.sub.10 -type, to a microwave source in the form of a magnetron. On the one hand this antenna can be regarded as an antenna in a conventional sense, i.e. as an element that can be described as having a radiation diagram, and on the other hand as a structure cooperating with the resonant modes in the cavity in a variable manner and thereby providing a varying coupling between the microwave input power and the resonant modes in dependence on the antenna position. In both cases, the result is that a number of different resonant modes are excited in succession, and that--in particular for large loads in the oven when the figure of merit (Q-factor) for resonant modes can be so low that it is not appropriate to consider the field as being resonant--a direct radiation into the load takes place.
In order to achieve a good microwave energy distribution in the oven cavity and more uniform heating of the load it has also been proposed that microwave energy be fed into the cavity at two spatially separated feeding points, an arrangement known as "dual feed". Reference may be made to the following prior art documents:
U.S. Pat. No. 3,939,320 describes an energy feeding arrangement that includes a resonant coupling structure disposed near a centrally located feeding point in the top of the oven cavity. This resonant structure is in the shape of a short hollow cylinder of conductive material which rotates eccentrically to spread an energy beam in many directions and which is contained in a "feed box" to produce energy reflections and facilitate protection by means of a cover permeable to microwaves.
U.S. Pat. No. 4,133,997 describes an arrangement for feeding energy to the oven cavity by means of rotating stirrers contained in "feed boxes" that are mounted to each of two opposite side walls. This "dual feed" arrangement is used to achieve a more uniform heating of the load in the oven cavity. Microwave energy is supplied to the feed boxes through a waveguide from a single magnetron resulting in an arrangement that is complicated and expensive.
U.S. Pat. No. 3,993,886 describes another type of dual feed arrangement including a flared waveguide into which microwave energy from a magnetron is fed at its smaller end. Microwave energy from several resonant modes of the TE-type is fed from the flared waveguide into the oven cavity through two coupling apertures at its wider end, in which apertures small rotating stirrers are placed for further improving the energy distribution in the oven cavity. U.S. Pat. No. 4,249,058 describes a similar dual feed arrangement that utilizes, however, rotating stirrers of a considerably larger diameter. An attempt at realizing a dual feed arrangement of more compact dimensions is found in U.S. Pat. No. 3,439,143. This arrangement comprises a rectangular waveguide to feed microwave energy from a magnetron to the oven cavity through two coupling apertures on both sides of a reflector of conductive material located within the waveguide and mounted off-axis to a rotating support of material that is transparent to microwaves. However, this compact arrangement does not achieve the advantage of that according to U.S. Pat. No. 3,993,886 producing several simultaneously existing resonant modes in the waveguide, nor the advantage of the arrangement according to U.S. Pat. No. 4,249,058 having rotating stirrers covering a substantial area of the top wall of the oven cavity.
U.S. Pat. No. 3,814,890 describes an arrangement for feeding microwave energy from a magnetron directly into the oven cavity without using an intermediate waveguide. This arrangement comprises a large feed box in the shape of a truncated cone having an aperture at its truncated end through which the radiating portion of the magnetron extends, but it does not comprise rotating antennas or stirrers for improving the energy distribution in the oven cavity.